The invention relates to a device for optical measurement, e.g. measuring turbidity, solids content and photometric sensing in media, more particularly fluids such as e.g. water and waste water. Since this very often involves measurements in heavily polluted waters there is a problem in cleaning, and maintaining clean, the measuring instruments. Dirt collecting on the measurement window detriments the measuring accuracy, it being particularly in long-term measurements that soilage of the measurement window may result in the sensed values becoming increasingly divorced from the true values. This is caused, for one thing, by biological growth, i.e. biofilms forming on the measurement window, and for another, also by such physical effects as e.g. window sedimentation and adhesion.
Known cleaning means work in making use of wipers which sweep the measurement window. This has, however, the disadvantage that the measurement window becomes smeared and dirt sticking to the wiper. In addition, it is not possible to make a measurement during cleaning which is unfavorable for long-term measurements.
Another method uses a jet of water for jetting the measurement window clean. This fails, however, to fully prevent the measurement window from becoming fouled. In addition dirt is prone to collect on protruding means such as e.g. nozzles and supporting elements. But, above all, this method introduces clean water into the medium being measured which directly influences the measurement results.
It is furthermore known in general to employ ultrasound for cleaning objects. Thus, e.g. U.S. Pat. No. 4,441,796 discloses a film transport assembly in which a lens located in the path of the optical beam is caused to vibrate in the ultrasonic range to remove dirt from the lens. However, the assembly as shown is much too bulky and ineffective to achieve good cleaning for other purposes.
It is thus the objective of the invention to provide a device of the aforementioned kind which permits non-falsified optical measurements without the measurement window becoming dirty.
This objective is achieved by a device or assembly as set forth in claim 1. Advantageous further embodiments of the invention form the subject matter of the sub-claims.
In accordance with the invention the device comprises a more particularly water-tight housing formed e.g. by a cylindrical beaker. In the region of one outer side of the housing, e.g. the face an optically transparent element, i.e. a measurement window is provided to permit directing the beam path of a sensor from/into the housing. The vibrator is arranged between the sensor and the measurement window. It contains at least one hole or opening for passage of the beam. As an alternative the vibrator may also be formed by several parts between which the incident/emerging beam of the sensor is guided.
This configuration in accordance with the invention achieves a highly compact and dense configuration of the measurement device suitable for measurement on-site. By arranging the vibrator between the sensor and the measurement window it is now possible to combine the vibrator and measurement window into a compact block without exposing the vibrator to the fluid being investigated. On the other hand, this assembly allows the sensor to be located spaced away from the measurement window and thus acoustically damped relative to the vibrator block of the vibrator and measurement window. This damping may be achieved by a reduction in the wall thickness of the housing and/or by locating damping material between the vibrator and sensor. By providing corresponding openings inline in the sealing material and vibrator it is now possible to steer the measurement beams as desired without the overall design becoming too complicated. The assembly in accordance with the invention thus permits effective mechanical coupling of a rugged vibrator, preferably a piezoceramic disk, with the measurement window whilst achieving a good mechanical decoupling of all vibrating components from the sensor. This results in an extremely reliable mode of operation permitting effective cleaning of the measurement window even during the measurement without deterimenting the measuring accuracy of the sensor. In addition to the compact design extremely highly reliable operation is assured by the assembly in accordance with the invention.
When the housing is configured to advantage as a cylindrical or beaker-shaped element, the measurement window is preferably configured as a face pane. The pane may be secured to a frame in the housing and/or to a ledge thereof to which the vibrator is then likewise coupled in a mechanical solid or rigid connection. It is, however, just as possible to fixedly connect the vibrator to the housing and to secure the measurement window to the vibrator. In any case a sealed joint between the measurement window and housing needs to be assured. The vibrator works preferably in the ultrasonic range, i.e. between 2 and 100 kHz, it preferably being arranged in a Faraday cage shielded from the sensor. The cage may be formed by the wall and ledges of the housing and/or solely by the housing walls and contact pads of the vibrator or of the sensor which are then required to extend over the full face. It is in this way that the electronics of the sensor are shielded from interference radiation which in turn adds to the resolution, reliable operation and accuracy of the measurement assembly.
Preferably, between the vibrator and the optically transparent element a housing diaphragm is configured defining both the vibrator and the optically transparent element. Securing the two components to the diaphragm of the housing is of advantage from a jointing point of view since both components require to be defined differingly by the housing. The optically transparent element, in other words the measurement window, can be positively frame-clasped by the housing and/or preferably likewise bonded full-surface to the diaphragm. Due to the consolidation of vibrator/diaphragm/measurement window a vibrator unit is now available which is easy to decouple from the remaining housing (damping reduction in water-tight housing) whilst being highly effective in preventing soilage of the measurement window and avoiding sensing and analysis being detrimented by the vibrations due to the unit being well damped from the other components.
A particularly compact design and advantageous mode of operation is achievable by arranging for the beam emerging from the housing and the scattered or reflected radiation to be at a specific angle to each other and relative to the housing; incident and emerging beam thus being at an angle in the range 10 to 120xc2x0 on having left the measurement window.
The measurement assembly as described above is particularly suitable for waste water turbidity measurement in which sedimentation, i.e. the build-up of sediments and the formation of biofilms is appreciably diminished. The assembly is suitable however for all optical measurements in fluids such as e.g. water and waste water.